W TERMINAL
A separate low voltage output terminal on the alternator which is used to operate a tach or RPM warning light.
definition from:
http://www.wagneralt.com/lines/glossary.asp?OpMode=Term&TermID=170
Why didn't MB use the alt as the tach signal source?
http://www.peachparts.com/shopforum/showthread.php?t=261289
It sounds like your car doesn't use it. So is there a risk of the exposed terminal shorting out on something? do you need to cover it in some way to protect it from road salt, etc?